This application is a continuation of U.S. application Ser. No. 10/721,104, filed Nov. 26, 2003, now abandoned, which is a continuation of U.S. application Ser. No. 09/831,524, filed May 10, 2001, now abandoned, the disclosures of which are expressly incorporated herein by reference. U.S. application Ser. No. 09/831,524 is a national stage filing under 35 U.S.C. § 371 of international application No. PCT/IB99/01810, filed Nov. 10, 1999, which published in the English language.
The invention relates to an optical component containing a normally hidden image.
A particular use of the components according to the invention is in the field of protection against forgery and copying and simple yet unambiguous document authentication.
The increasingly high-quality copying techniques which are becoming available make it increasingly difficult to safeguard banknotes, credit cards, securities, identity cards and the like against forgery. Furthermore, counterfeit branded products (even including counterfeit pharmaceuticals) and copies of copyright-protected products, for example compact discs, computer software and electronics chips, are being produced and distributed worldwide. The increasing number of forgeries necessitates new authentication elements which are safeguarded against forgery and can be identified both visually and by machine.
In the field of copy-protecting banknotes, credit cards etc., there are already a considerable number of authentication elements. Depending on the value of the document to be protected, very simple or relatively highly complex elements are employed. Some countries are content to provide banknotes with metal strips which come out black on a photocopy. Although this prevents them from being photocopied, elements of this type are very easy to imitate. In contrast to this, there are also more complex authentication elements, for example holograms and cinegrams. Authentication elements of this type are based on the diffraction of light by gratings and need to be observed under different viewing angles in order to verify their authenticity. These diffracted elements produce three-dimensional images, color variations or kinematic effects which depend on the angle of observation and have to be checked on the basis of predetermined criteria or rules. It is not practically possible to use machines for reading information, for example images or numbers, encoded using this technique. Furthermore, the information content of these elements is very limited, and only an optical specialist will be capable of discriminating definitively between forgeries and an original.
A further consideration with diffractive optical effects is that these have also been used for consumer articles such as wrapping paper, toys, and the like. The relevant production methods have therefore become widely known and are correspondingly straightforward to imitate.
Further to the diffractive elements mentioned above, other components are also known which are suitable for optimum copy protection. These include optical components, as disclosed, for example, by EP-A-689084 or EP-A-689065, that is to say components with an anisotropic liquid-crystal layer, which the latter has local structuring of the molecular orientation.
These components are based on a hybrid layer structure which consists of an orientation layer and a layer which is in contact with it and consists of liquid-crystal monomers or pre-polymers cross-linked with one another. In this case, the orientation layer consists of a photo-oriented polymer network (PPN)—synonymous with LPP used in other literature—which, in the oriented state, through a predetermined array, defines regions of alternating orientations. During the production of the liquid-crystal layer structure, the liquid-crystal monomers or pre-polymers are zonally oriented through interaction with the PPN layer. This orientation which, in particular, is characterized by a spatially dependent variation of the direction of the optical axis, is fixed by a subsequent cross-linking step, after which a cross-linked, optically structured liquid crystal monomer or pre-polymer (LCP) with a pre-established orientation pattern is formed. Under observation without additional aids, both the orientation pattern itself and the information written into the cross-linked LCP layer are at first invisible. The layers have a transparent appearance. If the substrate on which the layers are located transmits light, then the LCP orientation pattern or the information which has been written become visible if the optical element is placed between two polarizers. If the birefringent LCP layer is located on a reflecting layer, then the pattern, or the corresponding information, can be made visible using only a single polarizer which is held over the element. LPP/LCP authentication elements make it possible to store information, virtually without restriction, in the form of text, images, photographs, and combinations thereof. In comparison with prior art authentication elements, the LPP/LCP elements are distinguished in that the authenticity of the security feature can be verified even by a layman since it is not first necessary to learn how to recognize complicated color changes or kinematic effects. Since LPP/LCP authentication elements are very simple, reliable, and quick to read, machine-readable, as well as visual information, can be combined in the same authentication element.
However, there remains the risk that, in the course of time, forgers will be able to master this technique.
In the components described above, one pictorial element is present (whether visible or invisible).
It would be desirable to improve the security or entertainment value of such a component.
According to the present invention, an optical component comprises one or more retarder(s) in which are embedded a plurality of images, the images being so arranged that, at any point in the plane of the component, an element of not more than one image is present, each image being associated with a different interaction with polarized light. This association can be achieved in various ways, for example, each image is embedded in patterned retarder(s) (one or more), each of them having patterns with different optical axis.
The invention also provides a viewing system, comprising a source of polarized light, a component as set forth above, through which the polarized light can travel, and an analyzer (in practice, a polarizing sheet) for light which has traversed the component, the analyzer being rotatable about the axis of the direction of travel of the light. The source of polarized light may be a polarizing sheet applied to the surface of the component.
The different images may be contained in successive parallel stripes, each preferably narrower than the eye can resolve, into which the surface area of the component is divided; if these are n images, any one image will usually be represented on every nth stripe.
Such an optical component has the surprising property, valuable in the fields of entertainment, document authentication and forgery countermeasures, that simply by rotating a polarizer (=the analyzer), a plurality of different images, all visible in ordinary light, can be seen one after the other. Previously, it was possible to reveal only one hidden image in this way. However, all the advantages of the “one hidden image” technology, as described for example in EP-A-689065, PCT/IB98/00687 or CH 841/98, can be maintained.